With the development of electronics techniques, there has been a concomitant development of a need for digital display devices which can be read at all levels of light intensity, which consume but little power and which have long operating life times. Thus far, liquid crystals and light-emitting diodes have been used as the bases of such devices. The liquid crystal device suffers from the defect that the display produced is of low contrast so that it is difficult to read and, moreover, cannot be seen at all at low light levels. As for the light-emitting diode type of display, the power consumption is high which makes it difficult to use as a continuous display in small devices powered by a battery. Such devices are electronic calculators and wristwatches.
Recently, display cells based on electrochromic (hereinafter termed "EC") elements have been developed. Such display cells have low power consumption and provide a display which is readily visible at most light levels.
The principal difficulty with such EC devices is the relatively short lifetime as measured in display cycles. The difficulty becomes apparent in that reversal of the voltage intended to erase the image results only in partial erasure so that there is an after-image which remains visible.
As is evident, it would be highly desirable that a means of overcoming this difficulty with respect to the EC display be developed, in view of the fact that the power consumption of such a display is low and the visibility is high.